Nuclear Structure of 86Zr and 20O and Beam Pulsing Techniques

High angular momentum states in 86Zr were populated through the 58Ni(32S,4p) reaction at 135 MeV using the 88-Inch Cyclotron at Lawrence Berkeley National Laboratory. Recoiling 86Zr nuclei were stopped in a thick Ta backing. Prompt multi-γ coincidences with evaporated charged particles were detected using the full array of Gammasphere and the Microball. Mean lifetimes of 36 levels in 86Zr were measured using the Doppler-shift attenuation method. Transition quadrupole moments Qt were found in the range of about 0.3 to 1.5 eb in the positive-parity bands. The negative-parity bands show Qt values from about 0.25 to 1.2 eb In the yrast positive-parity band, a sharp drop in collectivity approaching the 30+ state supports the interpretation of band termination in this configuration. Decreasing Qt values approaching the 24+ and 27- states also provide an indication of terminating structures. Two beam pulsing devices were implemented at the Florida State University Superconducting Linear Accelerator Laboratory. The μs pulsing device consists of parallel plates which deflect the charged ion beam by applying a high voltage. Pulsing electronics controlling the beam deflection and acquisition system with variable time intervals were designed and built. This setup is suitable for lifetime measurements from about a hundred ns up to several ms. The ns pulsing device uses a 48 MHz buncher and two choppers operating at 24 and 6 MHz. This setup is suitable for lifetime measurements in the range of ≈ 1 to 100 ns. An extensive test experiment was performed to verify the accurate operation of these devices by populating known isomeric states with a wide range of lifetimes. Excited states in 20O were populated in the reaction 10Be(14C,α) at Florida State University. Charged particles were detected with a particle telescope consisting of 4 annularly segmented Si surface barrier detectors. The FSU γ detector array was used for γ radiation detection. Five new states were observed below 6 MeV from the α-γ and α-γ-γ coincidence data. Shell model calculations suggest that most of the newly observed states are core-excited 1p-1h excitations across the N = Z = 8 shell gap. Comparisons between experimental data and calculations for the neutron-rich O and F isotopes imply a steady reduction of the p-sd shell gap as neutrons are added.